1. Field of the Invention
The present invention relates to dielectric ceramic compositions and monolithic ceramic capacitors using the same.
2. Description of the Related Art
In general, a monolithic ceramic capacitor is produced as follows. Dielectric sheets are prepared. Each dielectric sheet is primarily composed of, for example, BaTiO3 and a surface is coated by an internal electrode material. These dielectric sheets are laminated, are thermally compressed, and are sintered to form a dielectric ceramic provided with internal electrodes. External electrodes in contact with each internal electrode are formed by baking on both side faces of the dielectric ceramic. The monolithic ceramic capacitor is thereby formed.
Materials used for the internal electrodes are noble metals, e.g., platinum, gold, palladium and alloys thereof, e.g., a silver-palladium alloy, because these metals are not oxidized when they are sintered together with dielectric materials. However, the use of such expensive metals is the largest factor in increasing the production costs of monolithic ceramic capacitors, although these electrode materials have superior characteristics.
Relatively inexpensive base metals, such as nickel and copper, have recently been used as materials for the internal electrodes; however, these base metals are readily oxidized in an oxidizing atmosphere at an elevated temperature, and these materials do not function as internal electrodes. When the sheets are sintered in a neutral or reducing atmosphere to prevent oxidation of the base metals for the internal electrodes, the dielectric ceramic layers are undesirably reduced and modified into semiconductor layers.
In order to solve this problem, for example, Japanese Examined Patent Application Publication No. 57-42588 discloses a dielectric ceramic composition comprising a barium titanate solid solution having a ratio of the barium site to the titanium site which is higher than the stoichiometric ratio, and Japanese Unexamined Patent Application Publication No. 61-101459 discloses a dielectric ceramic composition comprising a barium titanate solid solution and oxides of rare earth elements, such as La, Nd, Sm. Dy and Y.
Moreover, dielectric ceramic compositions exhibiting a small change in dielectric constant with temperature are disclosed. For example, a BaTiO3-CaZrO3-MnO-MgO-based composition is disclosed in Japanese Unexamined Patent Application Publication No. 62-256422 and a BaTiO3-(Mg,Zr,Sr,Ca) O-B2O3-SiO2-based composition is disclosed in Japanese Examined Patent Application Publication No. 61-14611.
Since these dielectric ceramic compositions are not reduced or modified into semiconductors during sintering steps in reducing atmospheres, the dielectric ceramic compositions facilitate use of base metals, such as nickel, as internal electrodes of the monolithic ceramic capacitors.
With recent trends toward miniaturization of electronic parts, miniaturization and increasing capacitance are also required for monolithic ceramic capacitors. The requirements for a dielectric ceramic composition used in such a monolithic ceramic capacitor includes a high dielectric constant, a small change in dielectric constant with temperature and a high insulating property for ensuring high reliability when the dielectric ceramic composition is used as thin films.
Conventional dielectric ceramic compositions are designed so that the monolithic ceramic capacitor may be used in low electric fields. When they are used as thin films, that is, when they are use in strong electric fields, some problems arise, such as significant deterioration of insulating resistance, dielectric strength, and reliability. When thin ceramic dielectric layers are formed using conventional dielectric ceramic compositions, the resulting monolithic ceramic capacitors must be used at lower rated voltages, depending on the thickness of the ceramic dielectric layer.
Although the dielectric ceramic compositions disclosed in Japanese Examined Patent Application Publication No. 57-42588 and Japanese Unexamined Patent Application Publication No. 61-101459 have large dielectric constants, the crystal grain sizes in the resulting ceramic are greater. When the thickness of one dielectric ceramic layer is decreased to 10 xcexcm or less, the number of the crystal grains which are present in each layer is significantly decreased, resulting in deterioration of reliability. Moreover, the resulting monolithic ceramic capacitors exhibit large changes in dielectric constant with temperature. Accordingly, these conventional monolithic ceramic capacitors do not satisfy market requirements.
The dielectric ceramic composition disclosed in Japanese Unexamined Patent Application Publication No. 62-256422 has a relatively high dielectric constant. Moreover, the resulting ceramic composite contains small crystal grains and exhibits a small change in dielectric constant with temperature. However, CaZrO3 and CaTiO3, which is formed during the sintering readily form a secondary phase together with MnO, etc. Thus, the resulting dielectric ceramic layer having a small thickness is less reliable at high temperatures.
The dielectric ceramic composition disclosed in Japanese Examined Patent Application Publication No. 61-14611 does not satisfy the X7R characteristic defined by the Electronic Industries Alliance (EIA) Standard, that is, a rate of change in electrostatic capacitance of within xc2x115% in a temperature range from xe2x88x9255xc2x0 C. to +125xc2x0 C.
In order to solve this problem, BaTiO3xe2x80x94Re2O3xe2x80x94Co2O3-based compositions, wherein Re is a rare earth element, are disclosed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068. Thin dielectric ceramic layers formed of these compositions, however, are still less reliable and do not sufficiently satisfy market requirements.
It is an object of the present invention to provide a dielectric ceramic composition suitable for dielectric ceramic layers of a monolithic ceramic capacitor which satisfies the B characteristic defined by the Japanese Industrial Standard (JIS) and the X7R characteristic defined by the EIA standard with respect to dependence of electrostatic capacitance on temperature, which has a relative dielectric constant (xcex5) of at least 2,500, a product (CR product) of insulation resistance (R) and electrostatic capacitance (C) when a voltage of 4 kVDC/mm is applied at room temperature of at least 5,000 xcexa9xc2x7F., and which has a prolonged service life with respect to insulating resistance in a high-temperature, high-voltage accelerated test.
It is another object of the present invention to provide a monolithic ceramic capacitor having improved reliability when the thickness of the dielectric ceramic layers is decreased.
According to an aspect of the present invention, a dielectric ceramic composition in accordance with the present invention comprises a compound oxide comprising barium titanate BamTiO3 as a major component and RO3/2, CaO, MgO, and SiO2, as accessory components, wherein R is at least one element selected from the group consisting of Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, wherein the compound oxide satisfies the following relationship:
100BamTiO3+aRO3/2+bCaO+cMgO+dSiO2
wherein, on a molar basis, 0.990xe2x89xa6mxe2x89xa61.030, 0.5xe2x89xa6axe2x89xa66.0, 0.10xe2x89xa6bxe2x89xa65.00, 0.010xe2x89xa6c less than 1.000, and 0.05xe2x89xa6d less than 2.00.
The compound oxide may further comprise a boron-containing compound as another accessory component in an amount not greater than about 5.5 moles on the basis of B2O3.
The compound oxide may further comprise a compound containing at least one element selected from the group consisting of Mn, Zn, Ni, Co and Cu as another accessory component, in an amount not greater than about 5 moles on the basis of the oxide MO of the element M.
The compound oxide may further comprise a compound containing Zr, Hf and at least one element selected from the group consisting of Ba, Ca and Sr as other accessory components, in an amount not greater than x7.0 moles on the basis of X(Zr,Hf)O3 wherein X is at least one of Ba, Ca and Sr.
According to another aspect of the present invention, a monolithic ceramic capacitor comprises a plurality of dielectric ceramic layers, internal electrodes formed between the dielectric ceramic layers, and external electrodes, electrically connected to the internal electrodes, wherein each of the dielectric ceramic layers comprises the above-mentioned dielectric ceramic composition and each of the internal electrodes comprises a base metal.